During the drilling of wells such as are used in the oil and gas industry, a drilling fluid is pumped through the well. The functions of this fluid are to carry drilled cuttings out of the borehole, to lubricate the drill bit, to balance the pressure of pore fluids in the drilled formations and to stabilise the wellbore wall, etc.
One important function is to control the well pressure and prevent downhole fluids from returning to surface. This is achieved by the hydrostatic pressure of the drilling fluid column which can be controlled by tuning the drilling fluid density. The hydrostatic pressure is selected to be higher than the pore fluid pressure (and so prevent pore fluid from entering the wellbore). However, if the hydrostatic pressure is too high, the pressure exerted on the wellbore wall might be so high as to fracture the formation. The range between pore pressure and fracturing pressure defines what is called the mud window. Once pressure reaches a value close to fracturing pressure, drilling is stopped and casing is placed and cemented to permanently isolate the formation from the wellbore. The drilling process can then start again with different conditions and a different mud window until it is necessary to set another casing. The number of casings needed for any given well will be dictated by the particular well conditions.
If the mud window can be widened by temporarily strengthening wellbore, the number of casings can be decreased and therefore the costs associated to well construction can be decreased. In addition, strengthening wellbore will reduce wellbore stability-related problems such as lost circulation or stuck pipe incidents, for example. This will translate into time savings and reduction of costs.
Finally, being able to reduce the number of casing strings by eliminating one or more of them may enable to drill deeper and reach reservoirs that could not be accessed due to the need for too many casing strings resulting in narrowing of the wellbore diameter to the extent that drilling cannot continue.
A wellbore may be strengthened during drilling by chemically modifying the mud filtercake, which is formed on the wellbore wall from mud mixed with the drilling fluid (which may contain additives) used during the drilling process.
Chemical modification of the mud filtercake will be dependant on the additives in the drilling fluid. For example, U.S. Pat. No. 4,768,593 and U.S. Pat. No. 4,760,882 disclose a drilling fluid which contains a polymeric material that, upon triggering by gamma rays or UV rays, can be crosslinked with a monomeric agent using a chemical initiator or a radioactive source, resulting in the conversion of drilling mud to cement.
U.S. Pat. No. 6,848,519 discloses the modification of the mud filtercake to form a chemical casing while drilling. The drilling fluid has a pH between 6 and 10 and contains a polymer and a water-dispersible thermoset resin that crosslinks the polymer. The drilling fluid also contains a particulate thermoset resin and a delayed dispersible acid catalyst that crosslinks the particulate thermoset resin. The chemical modification of the filtercake, which cures into a hard and tough crosslinked chemical casing, is induced by pH and temperature via curing of the thermoset resin and the use of the delayed dispersible acid catalyst.
Alternatively, instead of polymerization, cement hydration within a drilling fluid or filtercake, is also possible. U.S. Pat. No. 5,213,160, U.S. Pat. No. 5,476,144 and U.S. Pat. No. 5,464,060 disclose the addition of cement or blast furnace slag to a drilling fluid to provide a settable filtercake.
U.S. Pat. No. 6,204,350 discloses cure-on-demand, moisture-curable compositions, used as sealants, putties and adhesives, which contain an acid generating material that can be selected from carboxylate esters or sulfonate esters, as well as onium salts capable of curing the composition. The acid is released upon exposure to heat, ultraviolet light, visible light, electron beam irradiation or microwave irradiation.
Heat curable compositions used as seals, bonding materials and on tape, and that are controllably curable by microwave energy, are described in WO 01/28771.
US20060047028 discloses curing seal compositions containing a hydrogel polymer and a base material that can be polymerized in-situ by microwave irradiation, and EP0933498 discloses rapidly consolidating particulates material coated with hardenable resins in wells. U.S. Pat. No. 6,214,175 also describes recovery of gas hydrates using microwave radiation.
Microwave sources are also present in tools for measuring standoff from the borehole wall as is described in US 2006/0065394.
It is an object of this invention to provide techniques for strengthening the borehole wall during drilling and so extend the intervals between casing setting operations.